1. Field of the Invention
The present invention relates to a magneto-optical recording medium in which information is recorded, reproduced and erased by a light such as a laser beam. More specifically, the present invention relates to a magneto-optical recording medium having a metal reflecting layer.
2. Description of the Related Art
Optical recording media are being investigated and developed because of their high density recording and large storage capacity. Particularly, a magneto-optical recording medium is desired and various materials and systems therefor have been published because of its wide applicability to various fields.
A typical magneto-optical recording medium comprises a magneto-optical recording layer of an amorphous rare earth metal-transition metal alloy formed on a transparent substrate, the magneto-optical recording layer having an axis of easy magnetization perpendicular to the layer.
Since the magneto-optical recording layer of the amorphous alloy magnetic layer is easily oxidized and has a small Kerr rotation angle, resulting in an unsatisfactory C/N (carrier/noise) ratio of a reproduced signal, and so various proposals have been made to solve these problems.
For example, a four layer construction of a substrate/a transparent dielectric layer/a recording layer/a transparent dielectric layer/a metal reflecting layer in this order allows a utilization of Faraday and Ker effects together with a Kerr enhancement effect by the dielectric layer, thereby providing a high C/N ratio, and the four layer construction also provides a high durability by using the metal nitride as the dielectric layer. By providing a metal reflecting layer, the recording sensitivity is lowered in comparison with the case without the reflecting layer. To obtain an adequate recording sensitivity, there is a proposal to lower the thermal conduction of the metal reflecting layer by controlling the thickness of the metal layer or adding an additive to the metal layer.
Nevertheless, the present inventors found that in the above four layer construction, if the thermal conduction of the metal reflecting layer is made lower, the recording sensitivity is improved but several problems occur. For example, when information is being recorded, a large shift or dislocation of a recording bit from a position to be recorded occurs by a thermal influence between the neighboring bits. This bit shift can be detected as a shift of the peak position of the reproduced signal. The larger the shift of the bit is, the larger the peak shift is and more an error in reproduction occurs. Further, during erasing of information in which a high power laser beam is irradiated, the recording layer may be deteriorated by an over-heat if the construction of the medium does not allow a disperse of heat. In fact, in an inventors' investigation, the C/N ratios of media were often lowered when a continuous wave erasure laser beam (high power) was irradiated. Namely, there is a problem of a medium in a long term stability against a laser beam.
The object of the present invention is to solve the above problems and to provide a magneto-optical recording medium having the advantageous characteristics derived from the transparent dielectric layer and the metal reflecting layer while having a high C/N ratio and recording sensitivity as well as a low error rate of reproduced information and an excellent long term stability against a laser beam.